An initial seed is computed and stored in the game save file
Based on this seed, the world is pseudo randomly computed, using a certain chunk/block/tile size (e.g., 32x32, 128x128)
The world map is only generated on-demand, that is, as far as the player can see
When a new chunk is discovered, its blocks are computed and persisted in the save file
If there are no active components in a chunk that is not visible, the game will obviously not render it, it will only simulate it (position, item, velocity, etc)
Certain thing, for instance alien types and alien base size, may only be computed once they are observed. At first, maybe only a location anchor will exist to indicate where they should spawn, but the rest will be generated only when needed.

A table of probabilities is built
The ratio 1/drop chance is used to compute a total drop chance
A number is generated in the range 0-total drop chance
A table lookup is done to find the associated item
Item properties are randomly rolled
Different table lookup may be built depending on the difficulty setting as well as the current act
The rarity of an enemy pack may either change the random generator distribution or some other mean to modify the probability of higher quality items from dropping
The pseudo random number generator is initialized each game and does not depend on the current time (to avoid issue with reading some timer which may have the same value over many iterations or may be slow to read)
When an enemy is killed, we want to determine how many items will drop

Client/server
Per region servers
Login/authentication server
Lobby server
Store server

• buy champion/runes
  Transfer player from lobby to game server
• champion selection
• spectators
  Per game server
• Coordinates all 10 players within the game
• controls game events dragon/baron/npc/player gold
• compute damage
• end game lobby
  Game client
• Display animations
• play game state according to server

Authentication/Login server
Per game server

• compute damage simulation
• in game chat
• decide game victory
• returns end game stats for ui (or done client side?)
  Game client
• display game ui
• play animations
• send commands to game server
  Local backend
• record game
• compute game simulation
• communicate game state to game client

An initial seed is computed and stored in the game save file
Based on this seed, the world is pseudo randomly computed, using a certain chunk/block/tile size (e.g., 32x32, 128x128)
The world map is only generated on-demand, that is, as far as the player can see
When a new chunk is discovered, its blocks are computed and persisted in the save file
If there are no active components in a chunk that is not visible, the game will obviously not render it, it will only simulate it (position, item, velocity, etc)